In many industries, like the petrochemical and chemical industries for instance, the processes employ reactors in which chemical reactions are effected in the components of one or more reaction fluids under given temperature and pressure conditions. Most of these reactions generate or absorb heat to various extents and are, therefore, exothermic or endotherinic. The heating or cooling effects associated with exothermic or endothermic reactions can positively or negatively affect the operation of the reaction zone. The negative effects can include, among other things, lower product production, deactivation of the catalyst, production of unwanted by-products, and in extreme cases damage to the reaction vessel and associated piping. More typically, the undesired effects associated with temperature changes will reduce the selectivity or yield of products from the reaction zone.
One solution to the problem of negative temperature effects has been the indirect heating of reactants and/or catalysts within a reaction zone with a heating or cooling medium. The most well known catalytic reactors of this type are tubular arrangements that have fixed or moving catalyst beds. The geometry of tubular reactors poses layout constraints that require large reactors or limit throughput.
Indirect heat exchange has also been accomplished using thin plates to define alternate channels that retain catalyst and reactants in one set of channels and a heat transfer fluid in alternate channels for indirectly heating or cooling the reactants and catalysts. Heat exchange plates in these indirect heat exchange reactors can be flat or curved and may have surface variations such as corrugations to increase heat transfer between the heat transfer fluids and the reactants and catalysts. Although the thin heat transfer plates can, to some extent, compensate for the changes in temperature induced by the heat of reaction, not all indirect heat transfer arrangements are able to offer the complete temperature control that would benefit many processes by maintaining a desired temperature profile through a reaction zone. Many hydrocarbon conversion processes will operate more advantageously by maintaining a temperature profile that differs from that created by the heat of reaction. In many reactions, the most beneficial temperature profile will be obtained by substantially isothermal conditions. In some cases, a temperature profile directionally opposite to the temperature changes associated with the heat of reaction will provide the most beneficial conditions. An example of such a case is in dehydrogenation reactions wherein the selectivity and conversion of the endothermic process are improved by having a rising temperature profile that reverses the normal adiabatic temperature gradient through the reaction zone. A specific arrangement for heat transfer and reactant channels that offers more complete control can be found in U.S. Pat. No. 5,525,311, the contents of which are hereby incorporated by reference.
Most catalysts for the reaction of hydrocarbons are susceptible to deactivation over time. Deactivation will typically occur because of an accumulation of deposits that cause deactivation by blocking active pore sites or catalytic sites on the catalyst surface. Where the accumulation of coke deposits causes the deactivation, reconditioning the catalyst to remove coke deposits restores the activity of the catalyst. Coke is normally removed from the catalyst by contact of the coke-containing catalyst with an oxygen-containing gas at a high enough temperature to combust or remove the coke in a regeneration process. The regeneration process can be carried out in situ or the catalyst may be removed from a vessel in which the hydrocarbon conversion takes place and transported to a separate regeneration zone for coke removal. Arrangements for continuously or semi-continuously removing catalyst particles from a bed in a reaction zone for coke removal in a regeneration zone are well known. U.S. Pat. No. 3,652,231 describes a continuous catalyst regeneration process which is used in conjunction with the catalytic reforming of hydrocarbons, the teachings of which are hereby incorporated by reference. In the reaction zone of U.S. Pat. No. 3,652,231, the catalyst is transferred under gravity flow by removing catalyst from the bottom of the reaction zone and adding catalyst to the top while reactants flow cross-currently through a radial flow bed. U.S. Pat. No. 5,073,352 describes a reforming reaction system adapted to move a compact bed of catalyst downward in reactant channels for on-stream catalyst replacement while a reactant stream flows cross-wise through the reactant channels and while heat exchange channels, interleaved with the reactant channels, provide indirect heat from a heating stream. It is also known from U.S. Pat. No. 2,550,727 to move catalyst through a compact bed of catalyst particles by withdrawing particles from the compact bed of catalyst particles while passing reactants through the compact bed of catalyst in a direction co-current to the direction of catalyst movement,
A phenomenon known as "pinning" inhibits catalyst transfer in many reactor arrangements. Pinning is the phenomenon wherein the flow of fluid at sufficient velocity can block the downward movement of catalyst. Pinning is a function of the gas composition, the gas velocity, the physical characteristics of the catalyst, and the physical characteristics of the flow channel through which the catalyst must move. As the gas flows through the channels that retain the catalyst, the gas impacts the catalyst particles and raises intergranular friction between the particles. When the vertical component of the frictional forces between the particles overcomes the force of gravity on the particles, the particles become pinned. As the flow path length of gas through the catalyst particles becomes longer, the forces on the particles progressively increase from the outlet to the inlet of the flow channel. In addition, as the catalyst flow channel becomes more confined, the gravity flow of catalyst particles becomes more hindered. Accordingly, as the size of the flow channel becomes more confined, wall effects increasingly add to the vertical hold-up force on the catalyst particles. As a result, narrow flow channels have a greater susceptibility to pinning and cannot normally provide continuous catalyst circulation.
In the case of reactors providing indirect heat exchange, the arrangement of the reactor exacerbates the problem of catalyst pinning. Increasing the number of channels by decreasing their size facilitates heat transfer by increasing the surface area between the heat exchange fluid and the catalyst. In addition, heat transfer is further facilitated by irregularities in the plate surface that create turbulence and reduce film factors that interfere with heat exchange. However, irregularities in the plates that define the channels further interfere with the movement of catalyst and promote a greater tendency for the catalyst to "pin". Therefore, methods and reactor arrangements are sought to use a channel-type reactor that facilitates heat exchange and catalyst circulation while the reactor continues operation.
It is known to avoid some of the complexities of on-stream catalyst circulation by using a combination of fixed bed and moving bed reactor systems in series to treat a reactant stream. Combinations of series flow reactant streams that pass reactants through a fixed bed upstream of a moving bed system, downstream of a moving bed system, or have parallel trains of moving bed and fixed bed systems are shown in U.S. Pat. Nos. 5,417,843; 5,196,110; 5,190,639; 5,190,638; and 4,849,092. None of these references discusses the possibility of incorporating heat exchange into any of the reaction arrangements discussed therein.
Accordingly, it is an object of this invention to provide a process for the contact of reactants with a bed of catalyst while providing indirect heat exchange with a heat exchange fluid and on-stream circulation of the catalyst.
It is a further object of this invention to provide a simple reactor apparatus for indirect heat exchange of a reactant stream and for contact of the reactant stream with a bed of catalyst while obtaining advantages that attend on-stream circulation of the catalyst.